There's no continuity flaw! In the first shot, whole sections of the railing go away, but the first chunk the truck misses remains mostly intact. You can see in the second shot that the railing starts farther along the triangular piece of concrete than in the first shot.

BTW--they are running towards the truck because, well...THEY'RE ZOMBIES. They're STUPID. All they know is that there's MEAT in that truck. They don't care that it's gonna squash them like bugs. Zombies are dumb like that, y'know.